Efficient Removal of Lead (II) and Cadmium (II) from Aqueous Solution Using Eggshell-derived nano-calcium oxide: Kinetics, Isotherms, and Statistical Optimization

Abstract

Nano-calcium oxide (CaO) was synthesized from waste chicken eggshells by calcination. The synthesized nanoparticles were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), electron microscopy, N₂ adsorption–desorption analysis and zeta-potential measurements. The cubic nanoparticles had a crystallite size of 24 nm, calculated from the XRD data using the Scherrer equation, and a specific surface area of 85.4 m² g⁻¹. Pb(II) and Cd(II) were removed most efficiently at pH 5.0–6.0 (initial metal concentration 100 mg L⁻¹, adsorbent dose 0.6 g L⁻¹, 298 K), reaching equilibrium within 60 min. Adsorption followed pseudo-second-order kinetics (R² > 0.999) and the Langmuir isotherm, with maximum capacities of 242.7 mg g⁻¹ for Pb(II) and 186.3 mg g⁻¹ for Cd(II) at 298 K. The process was spontaneous and endothermic. Box–Behnken optimization (pH 6.2, dose 0.65 g L⁻¹, contact time 55 min) achieved more than 99% simultaneous removal, and the adsorbent retained about 90% of its capacity after five regeneration cycles. Removal proceeds mainly through ion exchange, surface precipitation and complexation, making eggshell-derived nano-CaO a sustainable, high-performance adsorbent for heavy-metal-contaminated water.